1. Field of the Invention
The present invention relates to a master disk used as a matrix for manufacturing a stamper by transferring, which is used for reproducing an optical disk made of for example a resin.
2. Description of the Prior Art
FIG. 1 is a process chart showing the conventional manufacturing process of the conventional optical disk;
FIG. 2 is a plan view showing the conventional master disk manufactured by the manufacturing process shown in FIG. 1; and
FIG. 3 is a plan view showing the conventional stamper made of a metal manufactures by transferring the master disk shown in FIG. 2 by electroforming.
Referring now to FIG. 2, reference numeral 1 designates a master disk formed by recording the information which has been previously compiled by the laser cutting, on an original regist plate formed by applying a photoregist on a surface of a polished glass plate. The master disk 1 is provided with a recording track periphery 2, on which the informations are recorded, and a recording track inside circumference 3, on which the informations are recorded likewise, formed concentrically.
In addition, referring to FIG. 3, reference numeral 11 designates a metallic stamper obtained by transferring a pattern of the master disk 1 by electroforming with the master disk 1 as the matrix and cutting. The stamper 11 is provided with a transfer recording track periphery 12, on which the recording track periphery 2 is recorded, and a transfer recording track inside circumference 13, on which the recording track inside circumference 3 is recorded. In addition, reference numeral 14 designates a center hole formed in a central portion of the stamper 11.
The manufacturing process of the master disk 1, the stamper 11 and the optical disk is described below with reference to FIG. 1.
At first, the glass plate is machined to form a disk having an appointed diameter and a center is positioned and then, the surface is polished followed by washing and drying. And, the photoregist serving as a recording film is applied on this surface and then, the applied photoregist is subjected to a heat treatment to prepare the original regist plate.
The prepared original regist plate is inspected on a defect of the recording film and film thickness and the original regist plate, which passed an inspection, is placed on a turntable so that a center of the original regist plate may coincide with a center of the turntable. Subsequently, the original regist plate is subjected to an exposure to a laser beam modulated in correspondence to the information signal, which has been previously compiled, that is, a laser cutting with rotating. Then, the development and etching are carried out to form the master disk 1 on which a pattern of the information signal is sensible with exposed portions and non-exposed portions as an uneven relief. Accordingly, the center of the recording track periphery 2 and that of the recording track inside circumference 3 of the master disk 1 coincide with a rotation center of the master disk 1 and a spiral recording track is formed between the recording track periphery 2 and the recording track inside circumference 3 with the rotation center of the master disk 1 as a fixed point.
Subsequently, this master disk 1 as the matrix is made electrically conductive on the surface thereof and then, nickel is electroformed on the surface of the master disk 1 to prepare the metallic original plate on which the pattern of the master disk 1 has been transferred. Then, in order to obtain the stamper meeting a metallic mold of an injection molding machine from this original metallic plate, the center hole 14 is formed and a periphery of the center hole 14 is subjected to a cutting process. The formation of this center hole 14 and the cutting process of the periphery of the center hole 14 are carried out by punching using a press, shearing, cutting by the use of a laser, cutting out by the use of a lathe but it is required that the center hole 14 is concentric with the rotation center of the master disk 1 in accordance with the center of the recording track transferred from the master disk 1. Accordingly, the center hole 14 is formed in high accuracy so that the center of the center hole 14 may coincide with the center of for example the recording track periphery 12 transferred from the master disk 1 and an eccentricity between the center of the formed center hole 14 and the center of the recording track transferred form the master disk 1 is strictly controlled.
The stamper 11, which has been finished through the above described manufacturing process and passed an inspection, is placed in a metallic mold of the injection molding machine to mold a resin. Thus, the optical disk made of resin having the quite same pattern as the pattern of the master disk 1 transferred on the stamper 11 is reproduced.
With the above described master disk 1, since it is required that the center of the center hole 14 and that of the recording track are concentric to each other when the center hole 14 of the stamper 11 is formed, the highly accurate cutting process, in which the center of the center hole 14 coincides with for example the center of the recording track periphery 12, must be carried out, that is, an advanced skill is requied. In addition, a precise operation must be carried out for a long time using a precision length measuring machine when a tolerance of diameter of the center hole 14, the eccentricity between the center of the center hole 14 and for example the center of the recording track periphery 12 and the like of the stamper 11, which finished the formation of the center hole and the cutting process of the periphery, are measured. In addition, problems have occurred in that individual variations are to be included in the judgement of the periphery of the cut stamper 11 and the edge portion of the center hole 14 in addition to the above described problems.